The use of ultraviolet (UV) radiation for the purpose of disinfection of a fluid, including liquids and gases, is well known. The process of using ultraviolet radiation to inactivate microbial contaminants in fluids is referred to as Ultraviolet Germicidal Irradiation (UVGI). Ultraviolet radiation has also been used for oxidizing organic and inorganic materials in a fluid, termed Advance Oxidation Process (AOP), and many commercial AOP systems are in use today. Systems employing UVGI and AOP methods rely on the ability to transmit UV radiation into the fluid in a predictable manner. The dose of a UVGI system, which has units of J/cm2, can be simply stated as the product of the UV irradiance in units of W/cm2 and the exposure time in seconds.
Both AOP and UVGI require a UV source. For practical purposes, the output irradiance of the UV source should be maintained and decay in a predictable manner over the usage lifetime of the UV source. This enables predictions about the replacement cycle of the UV source as well as the overall performance of the system. UV disinfection systems are typically specified for a certain performance level using a variety of factors, including Reduction Equivalent Dose (RED), End of Lamp Life (EOLL), Ultraviolet Transmittance (UVT) of the fluid, and Fouling (lamp window and reactor).
Some NSF and EPA regulations require UV disinfection systems to be tested with the UV source operating at predicted EOLL optical output power. In order to adhere to the UV disinfection system performance specifications for a predicted time period, the UV source should decay in a predictable manner. There are also commercial benefits to having longer EOLL, which leads to longer system lifetimes and/or UV source replacement intervals.
There are many types of UV sources. Historically, low pressure mercury vapor lamps, medium pressure mercury vapor lamps, and amalgam lamps have been used as UV sources for disinfection applications. Other UV sources include deuterium lamps, light emitting diodes (LEDs), lasers, micro plasma sources and solid-state field effect phosphor devices. Micro plasma lamps operate on the same principle as the large gas discharge lamps but have a planar electrode generating small localized pockets of UV emission. Solid state sources such as LEDs create light in a semiconductor material though charge recombination in an active layer where charge injection is applied to an anode and cathode of the semiconductor heterostructure. All of these UV sources have different optimal operating temperatures where the UV output flux and/or the lifetime is maximized. Most gas discharge lamps are difficult to operate in very cold ambient conditions because of the lower mercury vapor pressure. Conversely, solid state sources have maximized outputs at lower ambient temperatures. For example, the output power of a low-pressure mercury lamp may peak at an ambient temperature of 40 degrees Celsius while the optical output power of a 265 nm LED displays a linear relationship with ambient temperature. The slope of the LED curve may vary by the device design, but the trend remains the same with larger optical output powers seen at lower ambient temperatures.
Many LED manufacturers specify a maximum junction temperature which should not be exceeded. The LED junction temperature is the temperature of the active layer sandwiched between the n-type and p-type semiconductor layers of the LED. Exceeding a maximum rated junction temperature may result in a decrease in the lifetime or other characteristics of the LED. In a simplified model, an LED can be represented as a series of thermal resistances. For example, a UV LED package may be a surface mount device (SMD) mounted onto a circuit board, which is itself mounted onto a heatsink or other cooling device. The heatsink may be any heat exchanger or method of cooling, such as a passive heatsink, Peltier device, active airflow, heat pipe, etc. The LED may be mounted on a variety of electrically and thermally conductive circuit boards, such as a printed circuit board (PCB), a metal core printed circuit board (MCPCB), or a chip on board (COB). Every point of connection from the junction of the LED itself to the ambient environment has a temperature associated with it. These include the junction temperature of the LED, the temperature between the LED package at the circuit board, the temperature between the circuit board and the heatsink, and the ambient temperature. At each point of connection, one can model a thermal resistance, such that RJS is the thermal resistance of the surface mount LED packaged, RSB is the thermal resistance of the circuit board, and RBA is the thermal resistance of the heatsink or cooling method. The LED junction temperature can be modeled as the ambient temperature added to the sum of each of the thermal resistances multiplied by the power lost to heat in the device. This relationship is shown in Equation 1.TJ(LED)=TAmbient+Σi(Ri×PHeat)  Equation 1
LEDs are unique among most UV sources in that heat is removed through the side of the chip which is electrically connected versus the side which is responsible for most of the UV emission. This is in contrast to a mercury vapor lamp, which has a thermal discharge predominantly in the same direction as light emission through a quartz sleeve, which also functions as the arc discharge tube. LEDs do not require a quartz window as they emit light directly from the active layer of the semiconductor, and the light transmits through the subsequent layers of the semiconductor to exit to the ambient. However, LEDs can be sensitive to electro-static discharge, moisture, and ambient gases like oxygen or nitrogen which can degrade the performance of the LED electrical contacts and the semiconductor. For this reason, a quartz window is often placed on the SMD package of a LED. In UVGI systems where the LED will be protected from the fluid via a window, the window on the SMD becomes superfluous if the above environmental impacts can be mitigated. A single window over a board containing one or more LEDs can be used as the optical window for a fluid disinfection system if the LEDs are sealed between the board and the window such that the window can serve as a portion of the pressure vessel for the disinfection system and to segregate the LEDs from the fluid. Potting compounds like epoxies or silicones can be used between the board and the window to accomplish this. The potting may be undertaken in a low relative humidity environment or even purged with dry air or an inert gas to ensure any voids between the LED and window do not have undesirable moisture or gases inside. This would also increase the output power of the LED since it would pass light through one quartz window versus two. An additional benefit to this type of single window lamp package is that the LED imparts little heating to the window, in contrast to mercury vapor sources which transmit a large amount of heat to the window. Lower window temperatures have been correlated to less fouling of the window. Window fouling lowers the overall UV transmittance of the window, which in turn lowers the performance of UVGI and AOP systems. Thus, a robust product design utilizing a UV source will account for the temperature of the UV source during operation by consideration of heat transfer. By such methods the lifetime and output power of the UV source may be better controlled. In addition, methods of assembling the UV source into secondary packaging can be used to enhance the output power and lifetime of the UV source.
While the UV source is an important component in a UVGI system, it is only one component in the overall system efficiency. The system efficiency can be expressed as the product of the reactor efficiency and the UV source efficiency. It is good practice in the design of a UVGI system to maximize the exposure time, often termed the “residence time”, of the fluid to the UV irradiance thereby maximizing the dose seen by the fluid. The reactor efficiency is a combination of the residence time efficiency and the optical efficiency. The optical efficiency of the reactor is a measure of how effectively the reactor uses photons from the UV source to increase the probability that a microbial contaminant in the fluid will absorb a photon. One method of increasing this probability is to use reflective materials in the reactor such that photons from the UV source may be reflected if they are not absorbed during their initial pass inside the reactor. If there are few absorbers in the fluid and the reflectivity of the material in the reactor is high, the photons may be reflected multiple times inside the reactor.
U.S. Patent Application Publications 2012/0318749 A1, 2014/0161664 A1, and 2014/0240695 A1, all incorporated herein by reference, disclose various apparatus, materials and methods useful herein for disinfection of fluids by irradiation. However, what is still needed in the art is an improved apparatus and method for irradiation that provides good system efficiency, incorporates adequate thermal management, and can be used with a variety of housings or flow cells, all while maintaining a compact footprint.